Nonalcoholic fatty liver disease (NAFLD) is characterized by increased deposition of fat in the liver that may progress to inflammation, hepatocellular carcinoma, and liver failure. NAFLD is closely linked to type 2 diabetes mellitus and obesity. It remains a major public health problem with little progress made on its treatment. Disturbingly, NAFLD appears to be more common in the military and Veteran population compared with the general US population. Thus, efforts to understand the underlying mechanisms that contribute to NAFLD will provide valuable insight into developing therapeutic options for our Veteran population. Mitochondrial dysfunction plays a crucial role in the development of NAFLD, however, little is known about how to treat mitochondrial dysfunction and rescue the associated NAFLD. Mitochondrial trifunctional protein (MTP) is the major enzymatic complex in mitochondrial fatty acid oxidation (FAO) that consists of 4 ? and 4? subunits carrying three enzymatic activities breaks down fatty acids in the mitochondria. A key unanswered question is whether regulation of MTP modulates NAFLD. In a reported mouse model with an MTP defect, mice heterozygous for the defect (MTP+/-) develop mitochondrial dysfunction and are susceptible to NAFLD. Determining the mechanisms involved in regulating MTP in this mouse model is critical to the understanding of the role of mitochondria in development and rescue of NAFLD. The hypothesis in this application is that increasing the assembly of the MTP complex increases its stability, which leads to improvement in mitochondrial fatty acid oxidation (FAO) and rescue of NAFLD. The proposal in this application is that both the NAD+-dependent SIRT3 and the triiodothyronine (T3) hormone improve MTP stability by nongenomic mechanisms through enhancement of the MTP complex assembly. The preliminary data strongly support the hypothesis. First, mitochondrial FAO, MTP and SIRT3 levels were reduced in liver samples obtained from human subjects with NAFLD. Second, overexpression of SIRT3 in the MTP+/- mice reduced the acetylation of MTP compared to controls, increased mitochondrial FAO, and reduced steatosis and inflammatory markers in the liver. Third, in vitro studies in cultured cells demonstrate that MTP stability and its levels were increased by T3 treatment. To examine this hypothesis, we will use human liver samples obtained from a Veteran patient population and the MTP mouse model to conduct ex vivo, in vivo, and in vitro studies towards the following specific aims: 1) To define the relationship between hepatic MTP-? levels, SIRT3 levels, mitochondrial FAO and disease severity in human liver samples obtained from Veteran population. 2) To determine the underlying mechanisms of increased MTP levels by SIRT3 overexpression in mice. 3) To test whether T3 rescues NAFLD in the MTP mouse model and whether it improves hepatic mitochondrial FAO by increasing MTP stability through interaction with mitochondrial shortened thyroid receptors. This is the first study to examine regulation of MTP by SIRT3 and T3, and how that impacts mitochondrial FAO and NAFLD. This proposal addresses key unanswered questions and knowledge gaps in the field that impact a significant health problem in our Veteran population. Identifying the mechanisms by which SIRT3 and T3 regulate MTP will have important implications and will hold promise for future discovery of novel treatments for NAFLD, an increasingly common disease in Veterans with limited therapeutic options.